Vitamin D is produced by the body in response to exposure to sunlight but is also commonly found in fortified dairy products and supplements. It is already known to play a critical role in calcium absorption and bone health, but a growing number of studies suggest that vitamin D supplementation may also lower the risk of heart disease.

Researchers say vitamin D and calcium deficiency is a common problem among the elderly throughout the world. In the United States, the Institute of Medicine recommends vitamin D supplements at a dose of 400 to 600 International Units (IU) per day and calcium at a dose of 1,200 milligrams per day for adults over age 50.

But recent research suggests that significantly higher doses of vitamin D may be required to reap the maximum health benefits of vitamin D supplements.

To help clarify the role of vitamin D and calcium in heart disease risk, researchers analyzed 17 studies published between 1966 and 2009 on vitamin D and calcium supplementation and heart disease. The results appear in the Annals of Internal Medicine.

Researchers found six studies (five of which involved people on dialysis and one which included the general population) showed a consistent reduction in heart-related deaths among people who took vitamin D supplements. But four studies of initially healthy individuals found no differences in development of heart disease between those who received calcium supplements and those who did not.

A second analysis of eight studies showed a slight, but statistically insignificant 10% reduction in heart disease risk among those who took moderate to high doses of vitamin D supplements. No such reduction in heart disease risk was found among those who took calcium or a combination of calcium and vitamin D supplements.

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Researchers say very few studies have specifically investigated the effect of vitamin D supplements alone or in combination with calcium on heart disease risk in healthy people.

But evidence to date "suggests that vitamin D supplementation at moderate to high doses may have beneficial effects on reducing the risk for CVD [heart disease], whereas calcium supplementation seems to have no apparent effect," write researcher Lu Wang, MD, PhD of the division of preventive medicine at Brigham and Women's Hospital in Boston and colleagues.

They say more research is "urgently needed" to better explain the role of vitamin D in preventing heart disease.

The data should not be interpreted to mean that calcium supplements are harmful. Although calcium had only a neutral effect on heart health in the current analysis, calcium is known to be important for bone health. Calcium intake remains below recommended levels for a large portion of the U.S. population.